I. Field of the Invention
The present invention relates to electronic communications. More particularly, the present invention relates to Intermediate Frequency (IF) filtering.
II. Description of the Related Art
Electronic communication devices often modulate the desired signal onto a RF carrier frequency to provide frequency diversity over a plurality of channels. The distinct frequencies may then be simultaneously transmitted across a single link with a minimum of interaction between the plurality of channels. The link may be a single wire, multiple wires, coaxial transmission line, wireless link, optical fiber, or any other known communication link.
In a transmitter, baseband signals are upconverted onto a desired transmit frequency. While in a receiver, the received signal is downconverted to a baseband signal. The upconversion in a transmitter, and complementary downconversion in a receiver, is often performed in a plurality of stages rather than a single conversion.
Many communication devices utilize a dual conversion architecture design for the receiver and transmitter. FIG. 1 shows a block diagram of a wireless transceiver such as may be used in a wireless phone. Although a transceiver is shown in FIG. 1, it can be seen that the component parts may be isolated to perform only the transmitter or receiver functions. Similarly, although a wireless transceiver is shown in FIG. 1, a wire line device may be configured by eliminating the antenna or coupling the antenna to a wire line connection.
An antenna 10 may be used to interface the wireless device 100 to incoming radio waves. The antenna 10 may also be used to broadcast the signal from the transmitter. Incoming radio waves coupled to the wireless device 100 at the antenna 10 are next coupled to a duplexer 20. The duplexer 20 filters the incoming receive band signal but may also be used to electrically isolate the transmit power from the receive path while allowing the transmitter and receiver to use the same antenna. The duplexer 20 couples the signals in the receive path to a Low Noise Amplifier (LNA) 22 while simultaneously rejecting signals outside of the receive band. Ideally, the duplexer 20 rejects all signals in the transmit band such that they do not interfere with the receive band signals. However, practical implementations of duplexers 20 provide only limited rejection of signals in the transmit band.
The LNA 22 following the duplexer 20 is used to amplify the receive signal. The LNA 22 may also be the major contributor to the receiver""s noise figure. The noise figure of the LNA 22 adds directly to the noise figure of the receiver while the noise figure of subsequent stages is reduced in proportion to the LNA 22 gain. Thus, the LNA 22 is typically chosen to provide a minimal noise figure in the receive band while amplifying the receive signal with sufficient gain to minimize noise figure contributions from subsequent stages. There are competing design requirements, such as DC power requirements and device third order intercept point, that make the choice of LNA 22 gain a trade off of many design constraints. The signal amplified in the LNA 22 is coupled to an RF filter 24. The RF filter 24 is used to provide further rejection to signals outside of the receive band. The duplexer 20 may not be capable of supplying sufficient rejection of signals outside of the receive band so the RF filter 24 supplements the prior filtering. The RF filter 24 is used after, rather than before, the first LNA 22 stage in order to reduce the filter""s contribution to the receiver noise figure. The output of the RF filter 24 is coupled to a second LNA 26. The second LNA 26 is used to further amplify the received RF signal. A second LNA 26 stage is typically used where sufficient gain cannot be achieved in a single LNA stage while also satisfying third order intercept constraints. The output signal from the second LNA 26 is coupled to an input of a RF mixer 30.
The RF mixer 30 mixes the amplified receive signal with a locally generated frequency signal to downconvert the signal to an Intermediate Frequency (IF). The IF output of the RF mixer 30 is coupled to an IF amplifier 32 that is typically used to increase the signal level. The IF amplifier 32 typically has limited frequency response and does not amplify the upconverted signal that is output from the RF mixer 30. The output of the IF amplifier 32 is coupled to an IF filter 34.
The IF filter 34 is used to filter the IF from a single receive channel. The IF filter 34 typically has a much narrower frequency response than does the RF filter 24. The IF filter 32 can have a much narrower bandwidth since the RF mixer 30 typically downconverts the desired RF channel to the same IF regardless of the frequency of the RF channel. In contrast, the RF filter 24 must pass the entire receive band since any channel in the receive band can be allocated to the communication link. The output of the IF filter 34 is coupled to a receive Automatic Gain Control (AGC) amplifier 36. The AGC amplifier 36 is used to maintain a constant amplitude in the receive signal for the subsequent stages. The gain of the AGC amplifier 36 is varied using a control loop (not shown) that detects the amplitude of the amplifier""s output. The output from the AGC amplifier 36 is coupled to an IF mixer 40.
The IF mixer 40 downconverts the IF signal to a baseband signal. The Local Oscillator (LO) used in conjunction with the IF mixer 40 may be separate and distinct from the first LO 150. The baseband output of the IF mixer 40 is coupled to a baseband processor 102. The baseband processor 102 block represents subsequent processing that is performed on the baseband signal. Examples of subsequent processing include, but are not limited to, despreading, deinterleaving, error correction, filtering, and amplification. The received information is then routed to the appropriate destination. The information may be used within the wireless device or may be routed to a user interface such as a display, loudspeaker, or data port.
The same baseband processor 102 may also be used in the complementary transmitter. Information to be transmitted is input to the baseband processor 102 where it may be, for example, interleaved, spread, and encoded. The processed signal is coupled to a transmit IF mixer 110 where the baseband signal is upconverted to a transmit IF. The transmit LO 112 used in conjunction with the transmit IF mixer 110 is generated separately from the first LO 150 and the receive IF LO 42.
The upconverted transmit IF signal output from the IF mixer 110 is coupled to a transmit AGC amplifier 114. The transmit AGC amplifier 114 is used to control the amplitude of the transmit IF signal. Amplitude control of the IF signal may be required to ensure the signal is maintained within the linear regions of all subsequent amplifier stages. The output of the AGC amplifier 114 is coupled to a transmit IF filter 116 that is used to reject unwanted mixer and amplifier products. The filtered output is coupled to a transmit RF mixer 120. The transmit RF mixer 120 is used to upconvert the transmit IF to the proper transmit RF frequency.
The upconverted RF output from the transmit RF mixer 120 is coupled to a first transmit RF filter 122. The first transmit RF filter 122 is used to reject undesired mixer products. The output of the first transmit RF filter 122 is coupled to a driver amplifier 124. The driver amplifier 124 amplifies the signal to a level desired by the subsequent power amplifier 128. Before the signal is applied to the power amplifier 128 the signal is filtered in a second transmit RF filter 126. The second transmit RF filter 126 is used to further reject mixer products and is also used to reject out of band products that are generated by the driver amplifier 124. The out of band products generated by the driver amplifier 126 may be harmonic products generated by driving the amplifier into a non-linear operating range. The output from the second transmit RF filter 128 is coupled to a high power amplifier 128. The high power amplifier 128 is used to amplify the transmit signal to a power level sufficient to ensure a communication link to a recipient. The output of the high power amplifier 128 is coupled to an isolator 130.
The isolator 130 is used to protect the output of the high power amplifier 128. Signals from the high power amplifier 128 are able to pass through the isolator 130 with minimal loss but signals that are incident at the output of the isolator 130 are greatly attenuated at the input to the isolator 130. Thus, the isolator 130 provides a good impedance match to the output of the high power amplifier 128 and protects the amplifier from reflected signals due to impedance mismatches in subsequent stages. The output of the isolator 130 is coupled to the duplexer 20 that is used to couple the transmit signal to the single antenna 10 while simultaneously rejecting the transmit signal from the receive path.
A baseband processor 102 may be capable of processing signals having multiple formats. A transceiver operating in a communication system in accordance with Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) IS-95-B, MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM SYSTEMS must be capable of operating in analog mode as well as in digital Code Division Multiple Access (CDMA) mode. One problem associated with operating in multiple communication modes is the different IF bandwidth requirements of the different modes. In the IS-95-B specification, the analog channels operate in 30 KHz bands while the CDMA channels operate in 1.23 MHz bands. The different IF bandwidths may be accommodated using multiple IF filters, with a particular IF filter assigned to a particular mode. A switching circuit must be used to switch the appropriate IF filter into the signal path to correspond with the operating mode. However, the use of multiple IF filters and a switching circuit to accommodate multiple IF bandwidths of multiple operating modes is not ideal. An IF filter is required for each operating mode and additional modes require additional IF filters. What is needed is a programmable IF bandwidth that is capable of accommodating multiple IF bandwidths without the need for multiple filter configurations. The single configuration should be able to conform to multiple IF bandwidths required by multiple operating modes.
The present embodiments disclose is a novel and improved method and apparatus of providing a programmable IF bandwidth using fixed bandwidth IF filters.
In one embodiment two frequency conversions are used with a fixed bandwidth IF filter following each frequency conversion. The first Local Oscillator (LO) for the first frequency conversion is tuned such that the center frequency of the desired signal is converted to a frequency that is near one bandedge of a first IF filter. The spacing between the center frequency and the nearest bandedge is chosen to be one half of the desired IF bandwidth. Then the second LO used in the second frequency conversion is tuned such that the spacing between the center frequency and the nearest bandedge of a second IF filter is equal to one half the desired IF bandwidth. When the second LO is tuned such that the second IF filter rejects signals not filtered by the first IF filter, the result is a tunable IF filter having the desired bandwidth.
In a first embodiment the first LO converts the desired signal to a center frequency such that the frequencies above the center frequency are attenuated using a first IF filter. The second LO converts the desired signal to a center frequency such that the frequencies below the center frequency are attenuated using the second IF filter. The result is an IF bandwidth that is the result of the placement of the desired signal relative to the two IF filter bandedges.
The first and second IF filters may be bandpass filters, lowpass filters, highpass filters, or any combination of bandpass, lowpass, and highpass filters. The desired signal may be offset towards the upper or lower bandedge when bandpass filters are used. The desired signal must be offset towards the bandedge when either a highpass or lowpass filter is used. If a highpass filter is used only the frequencies above the center frequency may be attenuated. Similarly, when a lowpass filter is used only the frequencies below the desired center frequency may be attenuated. The terms above and below the center frequency are relative to the particular frequency spectrum since the frequency conversion may result in a spectral inversion.
The input signal is frequency converted to a first IF where a first IF filter establishes a first bandwidth of a desired IF signal. The signal output from the first IF filter is frequency converted to a second IF where a second IF filter establishes a second bandwidth of the desired IF signal. The first bandwidth of the desired IF signal may be the upper bandwidth denoting the frequencies above a desired IF center frequency. Similarly, the second bandwidth of the desired IF signal may be the lower bandwidth denoting the frequencies below the desired IF center frequency.